1. Technical Field
The present disclosure relates to a regulator for a charge pump and a circuit that implements the regulator.
2. Description of the Related Art
A charge pump is a particular boosting circuit that is used to generate higher voltage than the supply voltage thereof. Normally, charge pumps are implemented in integrated circuits. In general, the operation of a charge pump is based on removing a significant charge quantity from the supply voltage and transferring the charge quantity to a series of capacitive stages that convey the charge quantity to the output. The frequency of this removal is determined by a regulating circuit that enables the output voltage to be fixed at the desired value.
Current regulating circuits compare the output voltage of the charge pump and a reference voltage. On the basis of this comparison, the regulating circuits supply the enabling signals to the various stages of the charge pump to remove the charge from one stage to a successive stage of the series, with a frequency variable between zero and a clock frequency set externally. In FIG. 1 there is shown a charge pump 1 with a regulating circuit 2 having an input voltage Vout exiting from charge pump 1 and a clock signal CK. The signal Ckin exiting from the regulator 2 enters the charge pump 1 that is supplied by a supply signal Vdd.
This operating principle has the advantage of maximizing the efficiency of the pump as it is switched on only when necessary. The drawback arises from the fact that the frequency of the removal of current from the supply depends on the load attached to the pump; in fact, transmitting enabling signals to low-frequency capacitive stages generally corresponds to a loadless pump status, i.e., to the absence of a load, whilst transmitting enabling signals to the high-frequency capacitive stages corresponds to a high request from the load. In FIG. 2 there are shown the time diagrams of the signals Ivdd, Vout and Iout.
This pattern leads to variable frequency absorption spikes on the supply, and this generates a wide-spectrum noise that is reflected in the entire integrated circuit supplied by the supply voltage. In FIG. 3 there is shown the frequency spectrum S of the current Ivdd.
A charge pump in which there are provided means for reducing noise is known from patent EP 1408604. The charge pump comprises a sequence of capacitive stages and means for transferring the electric charge from each capacitive block to the next capacitive block of the sequence. The transferring means are enabled alternatively during a first phase and a second phase. The charge pump includes means for limiting the current that runs through each capacitive stage during the first phase and the second phase.